(a) Field of the Invention
This invention relates to a process for the production of 2-chloropropionaldehyde from vinyl chloride, carbon monoxide and hydrogen as raw materials.
(b) Description of the Prior Art
2-Chloropropionaldehyde can find applications as useful intermediates for chemicals, agricultural chemicals and medicines. It has been known to produce this compound from vinyl chloride, carbon monoxide and hydrogen as raw materials, as disclosed, for example, in French Pat. No. 1,397,779 and HELVETICA CHIMICA ACTA, 48(5), 1151-1157. All of these processes employ cobalt carbonyl as a catalyst. In French Pat. No. 1,397,779 referred to above, for example, the raw materials are reacted for 90 minutes under the conditions of a reaction temperature of 110.degree. C. and a reaction pressure of 200 atm., thereby obtaining reaction results of a vinyl chloride conversion of 57.4% and a selectivity toward 2-chloropropionaldehyde of 86.2%.
However, these processes which employ cobalt carbonyl as a catalyst require cobalt carbonyl in a large amount and a reaction pressure as high as 160-200 atm. because the catalytic activity of catalyst per unit amount of cobalt is extremely low. Moreover, the reaction is carried out at a reaction temperature of 75.degree.-125.degree. C. for 90-120 minutes.
The intended product, 2-chloropropionaldehyde, is a thermally-unstable material. Under the conditions of such a reaction temperature and reaction time, a substantial portion of 2-chloropropionaldehyde is consumed through a consecutive reaction and the reaction yield is thus reduced. Accordingly, these processes have poor reproducibility.
Further, hydrogen chloride is by-produced through the consecutive reaction or other side reactions. It causes the materials of reactor to suffer severe corrosion and it reacts with the cobalt carbonyl catalyst to form cobalt chloride. Therefore, the processes involve problems of developing an obstacle to the reutilization of the catalyst.
As improved processes in these disadvantages, the present inventors have found processes in which vinyl chloride, carbon monoxide and hydrogen are reacted in the presence of a rhodium compound and a base as disclosed in Japanese Patent Laid-Open Nos. 126046/1986, 10038/1987, 22738/1987 and 96444/1987. In accordance with these processes, the reaction proceeds at lower temperatures and pressures than in the conventional processes which employ cobalt carbonyl as a catalyst and a sufficient selectivity can be attained toward the intended product.
In these processes, a combination of a compound represented by the general formula P(R.sup.1 R.sup.2 R.sup.3) wherein P denotes a phosphorus atom and R.sup.1, R.sup.2, and R.sup.3 represent individually an alkyl, aryl, cycloalkyl, alkoxy, aryloxy or cycloalkoxy group and a nitrogen-containing compound the pka of which is in the range of 3-11 may preferably be used as the base in the presence or absence of water.
Among these nitrogen-containing compounds having pka levels of 3-11, pyridine compounds, quinoline compounds, imidazole compounds and morpholine compounds are used particularly favorably in view of reaction results and other aspects. All of these bases are comparatively expensive compounds. Therefore, it is necessary to minimize the loss occurring upon their industrial use by providing, for example, with a recovery apparatus of these bases.
Further, all of these compounds are highly reactive so that they are consumed, though slowly, when they are used for a prolonged period of time. It is therefore necessary to conduct the operation in such a manner as to prevent the loss to greatest possible extent. The conditions of the operation however do not necessarily agree with the conditions which are favorable to the synthesis of 2-chloropropionaldehyde. Accordingly, the processes involve problems in that the reaction under the conditions with a little deviation from the optimum synthesis conditions as well as the loss exerts considerable influences on the production cost of the intended product, 2-chloropropionaldehyde.
Further, there are many compounds which have vapor pressures among the pyridine compounds, quinoline compounds and morpholine compounds. These compounds are disadvantageous in that they are admixed into the reaction product of 2-chloropropionaldehyde, though in small amounts, upon separation of 2-chloropropionaldehyde from the reaction solution through distillation, thereby not only degrading the purity of the product of 2-chloropropionaldehyde but also interfering with oxidation reaction seriously upon the production of 2-chloropropionic acid by oxidizing 2-chloropropionaldehyde.